3D Printing in Microbial Fuel Cell

Abstract

The rise of additive manufacturing (AM), commonly known as 3D printing (3DP), is attributed to its ability to fabricate complex 3D structures swiftly and accurately from computer-aided design (CAD) models with minimal labor involvement. Given the heightened popularity in 3DP, researchers have explored its potential in microbial fuel cell (MFC) technology, utilizing it for the production of various MFC elements such as reactor bodies, separators, and membranes. Over the last decade, innovative electrode designs and cell arrangements have emerged, contributing to the enhanced performance of MFCs. This is largely owing to the capability of 3DP, allowing for individual optimization of each MFC component by facilitating independent design for reactors and components. Moreover, a significant attribute of 3DP technology lies in its consistent production capabilities, enabling the scalability of MFC systems by creating multiple stacks of MFC units while ensuring minimal material wastage and eliminating human errors. The forthcoming book chapter discusses the application of 3DP in MFCs.

1. Introduction

Microbial fuel cell (MFC) is the most researched technology among other microbial electrochemical technologies (MET). Owing to the extensive efforts accomplished by researchers, the primary purpose of generating electricity using MFC has now expanded to various applications, including wastewater treatment, bioremediation, desalination, energy conversion for green fuel synthesis, carbon sequestration, and biosensing [1, 2]. MFCs are bioelectrochemical systems (BESs) that utilize exoelectrogens as biocatalysts, which are electron-producing microorganisms that oxidize organic and inorganic components to produce electrons to the anode [3]. These electrons are then transferred to the cathode via the load to facilitate the oxygen reduction reaction (ORR). Till date, the MFC technology has reached a fascinating point, with enormous potential for practical applications, especially in pursuing the United Nations’ sixth (clean water and sanitation) and seventh (affordable and clean energy) sustainable development goals (SDGs). In order to realize such implementations on a larger scale, it is critical to improve the technology’s efficiency relating to the MFC’s performance, manufacturing and operating costs, stability, and lifespan [4]. This can be achieved by optimizing the design and fabrication process of each MFC component.

Additive manufacturing (AM), often known as 3D printing, is a fabrication technique that generates complex and sophisticated three-dimensional objects from computer-aided design (CAD) files [5]. Compared with conventional MFC modification techniques, AM enables a higher degree of design freedom and opens up various possibilities for the type of materials used [6], which is significant for the performance of electrodes and the durability of reactor bodies. Since exoelectrogens grow and form a biofilm on the anode, the anode material and its design have been extensively studied for the past two decades. In terms of electrode design, three-dimensional electrode structures possess higher performance compared to those of two-dimensional structures due to their higher surface area, lower internal resistances, lower activation energy, and lower mass transfer overpotential [7]; while stainless steel is the most feasible electrode material for MFC as other metals tend to corrode easily [8]. Therefore, a combination of carbon-based catalysts with stainless steel electrodes also enhances the biocompatibility of the MFC anodes. By controlling the structure design and printing material, novel MFC components, including electrodes, reactor bodies, and membranes, can be fabricated.

2. 3D printed MFC electrodes

Microbial fuel cell (MFC) electrodes with optimal biocompatibility for biofilm development and excellent electrochemical characteristics for high current density production have always been vastly pursued by MET researchers. Notably, anodes with 3D surface offer high surface area for efficient biofilm formation thanks to the easier access of substrate to the exoelectrogens, which remarkably reduces mass transfer restrictions [9]. Owing to AM’s flexibility, 3D electrodes with macroporous structures can be fabricated with ease. Based on Figure 1a, different sizes and designs of electrodes were fabricated using the fused deposit modeling (FDM) printing technique. These structures were printed layer-by-layer using conductive polylactic acid (PLA) filaments as printing materials. The researchers found that the electrodes had minimal degradation and fouling after conducting the experiment for 58 days in a plant MFC (PMFC) system [10], which proved that 3D printed electrodes do possess excellent physical and cycling stability [12]. Stackable PMFC electrodes were also fabricated and tested for their electricity generation efficiency. It was found that stacking the 3D printed PMFCs in series caused voltage loss while stacking the cells in parallel preserved the voltage and current [11], which means parallel stacks of PMFCs are more feasible for scaling up. Also, different unique designs and configurations of MFC can be achieved through 3D printing (Figure 1b).

Figure 1.

(a) Different sizes and designs of 3D printed conductive PLA electrodes. Reprinted with permission from Ref. [10] under an open-access creative commons CC BY-SA license. Copyright 2023, CBIORE. (b) Assembled PMFC with 3D printed anode and cathode. Reprinted with permission from Ref. [11] under an open-access creative commons CC BY-SA license. Copyright 2023, CBIORE.

Despite the superior mechanical properties of the 3D printed electrodes, conductive PLA as the sole electrode material still suffers from low electrical conductivity [10, 11]. Generally, modification of the electrode surface with suitable catalysts can enhance the electrochemical properties of the electrode. This was proven when nonconductive and conductive PLA anodes synthesized through FDM were both modified with carbon coating and tested for 25 days. The peak power generated by the carbon-coated nonconductive and conductive PLA anodes was relatively close, indicating that the performance was mainly attributed to the catalyst coating regardless of the electrode base material [13]. Following the success of surface modification on 3D printed electrodes, filament manufacturers were driven to develop composite filaments that consist of conductive PLA and other catalysts. Indeed, conductive PLA electrode infused with 8 wt% graphene powder were successfully printed via FDM and used in a P. aeruginosa MFC system. It was found that the graphene-containing conductive PLA electrode produced a power output of 110.74 ± 14.63 μW m⁻², which was comparable to the highly conductive carbon cloth electrode (93.49 ± 5.17 μW m⁻²) [14]. Apart from commercial FDM filaments, researchers have also tried to synthesize their own filaments with desired compositions. Carbon black powders were mixed with acrylonitrile butadiene styrene (ABS), which is a common 3D printing material and extruded into filaments using a commercial filament extruder. The carbon black-ABS anodes performed significantly better than the plain PLA anodes and exhibited comparable power outputs with the carbon cloth anodes [15]. This is due to the addition of carbon black powders, which greatly enhanced the biocompatibility of the electrodes.

Inspired by the customized filaments, researchers have discovered a more streamlined approach to 3D print composite materials without synthesizing the filaments. Through direct ink writing (DIW) method, composite inks in the form of viscous liquid can be extruded directly layer-by-layer using a dispensing machine with various adjustable nozzle sizes. A PTFE-free alginate-activated carbon electrode was synthesized by manual extrusion of the thick carbon-alginate paste (Figure 2) and left to solidify for 24 hours. The 3D printed PTFE-free alginate-activated carbon electrode marked the highest power output up to 268 μW as compared to the commercial carbon block (85 μW) and the non-3D printed PTFE-activated carbon (98 μW) when the MFCs were tested for 28 days [16]. Indeed, biofilm development on the electrode surface remains one of the most time-consuming processes in MFC. Recently, the direct printing of a living MFC anode was reported. Living bacteria (Shewanella Oneidensis MR-1) ink was prepared by mixing its growth medium with cellulose, acetylene carbon black, and sodium alginate. The as-prepared ink was then extruded through a nozzle to form the desired electrode design and cured using a calcium chloride solution to form a solidified calcium chloride matrix containing the bacteria. A peak current density of 9.233 μA cm⁻² was reached within 4 days upon stabilization of the MFC [17]. It was observed that there were only a small number of bacterial cells embedded in the electrode structure prior to any tests; after the stability test was carried out, the number of bacteria increased significantly while retaining their physical characteristics. This indicates that 3D printed live microbial electrodes are feasible for MFC technologies and the growth of the bacteria can be restored by providing sufficient nutrients.

Figure 2.

Direct ink writing of PTFE-free alginate-based carbon electrode under 1 minute of fabrication time. Reprinted with permission from Ref. [16] under an open-access creative commons CC BY license. Copyright 2020, MDPI.

Among other 3D printing techniques, digital light processing (DLP) has been drawing immense interest thanks to its rapid printing speed, flexibility in structure design, and high-resolution printing capabilities. DLP works by projecting the shape of an entire layer of a sliced-3D model on the printing platform and curing that entire layer at the same time before moving to the next layer [18]. In short, DLP prints 3D structures based on a projection layer-by-layer moving only on one axis (the z axis, up and down), while FDM and DIW fabricates by moving in all three axes (the x and y axis of a plane, followed by the z axis) to form the 3D structure. By using UV curable resins, 3D printed porous structures were fabricated and served as the base material for copper electroless plating. As a result, 3D printed porous copper electrode was successfully produced (Figure 3), which recorded a maximum voltage and power density of 62.9 mV and 6.45 mW m⁻², respectively, which were 8.3 and 12.3 times higher than the commercial copper mesh electrodes. However, the authors have identified the toxicity of copper ions to the Shewanella Oneidensis MR-1 culture after 15 days of operation, which hindered the performance of the MFC [19]. Despite the drawbacks of this research, the potential of DLP in MFC was shown and could be further explored with more biocompatible materials. This phenomenon was later realized when a 3D porous carbon structure with high biocompatibility was prepared by carbonization of a DLP printed 3D porous structure. In this work, 3D porous structures with various pore sizes were printed, mainly 100, 200, 300, 400, and 500 μm. The substitution of copper with carbon led to a maximum voltage and power density obtained was 453.4 mV and 233.5 mW m⁻² [20], respectively, which were 7.2 and 36.2 times higher than their DLP-printed-copper electrode counterpart.

Figure 3.

FESEM image of DLP printed anode structure, insert shows the actual high-resolution 3D printed carbon electrode. Reprinted with permission from Ref. [19] under an open-access creative commons CC BY license. Copyright 2018, Frontiers.

Another 3D printing technology similar to that of DLP that uses light projection is masked stereolithography (MSLA). MSLA creates the whole layer of the 3D structure as well, but instead of using a projector that projects light in a certain shape, MSLA utilizes an array of ultraviolet light-emitting diodes (LEDs) as its light source that remains unchanged regardless of the shape of the layer. The shape of the printing layer is formed by an LCD panel acting as a photomask that blocks light surrounding the shape where nothing is to be printed. One of the greatest benefits of MSLA is that it allows for high-resolution printing of complex structures rapidly, making it more suitable for large-scale applications [21]. By using MSLA, a 3D printed electrode decorated with graphene oxide (GO) and polypyrrole (PPy) was synthesized. Upon carbonization, average pore sizes of 0.4 mm were achieved for the electrode, which provided large specific surface area for biofilm development and ensuring efficient mass transfer of the electrode. When used as the MFC bioanode, the PPy/GO electrode recorded a high peak power density of 22.4 W m⁻², which was approximately 19 times higher than that of commercial carbon felt anodes [22].

In essence, 3D printing methods have demonstrated effectiveness in creating electrodes with enhanced structures and improved properties. Among the various 3D printing techniques, DIW and FDM methods stand out due to their flexibility and simplicity of operation. As 3D printing continues to evolve, it is anticipated that more sophisticated printing techniques will be discovered in electrodes fabrication. Lastly, 3D printing also represents a significant revolution in electrode fabrication. In contrast to the traditional 2D planar electrode structure, 3D printing offers a novel approach to address challenges in microbial electrochemical technologies by employing three-dimensional electrode architecture designs that greatly boost the performance of the MFC, as well as biocompatibility with the electron producing microbes.

3. 3D printed MFC reactor bodies

One of the greatest challenges of commercializing MFCs in practical applications is the scale-up of the reactor components such as reactor bodies, membranes, and electrodes as the inconsistent efficiency of MFCs is still unable to compensate for the huge capital costs. Furthermore, the scaling-up process for the practical implementation of MFCs requires substantial expenses mainly due to the materials used, namely platinum electrodes and Nafion membranes. Hence, researchers have strived to obtain a more stable energy production from MFCs and reduce their costs. For these reasons, scientists have explored the use of additive manufacturing to fabricate MFC parts, including the reactor bodies.

In the development of METs, biocompatible materials are always preferred as they provide better platforms for biofilm attachment and growth. By using FDM, nontoxic biocompatible PLA was used to construct the MFC reactors with a high level of accuracy up to 5 μm. It can be observed that the 3D printed single-chamber MFC reactor does not differ from the conventional acrylic fiber material in terms of built quality. Next, complementary reactor shapes can be further printed to construct double-chamber MFCs, indicating that the 3D printing has indeed opened up various potentials for reactor up-scaling or stacking MFC configurations. Conductive PLA with low resistance values (30 Ω cm⁻¹) was also fabricated in this study. It was found that both configurations were greatly affected by the high internal resistance, which hindered the power generation. However, the double-chamber MFC produced a better result compared to the single-chamber MFC, which was due to the use of cation exchange membrane (CEM) as well as potassium permanganate at the cathode side, which balanced the redox reactions [23].

Plant membrane fuel cells (PMFCs) show promising prospects for generating bioelectricity as they represent renewable and environmentally friendly energy sources. However, their capacity for power generation remains a challenge because their power output has not yet reached a level that makes them a cost-effective option for renewable energy. These lower power densities are a result of various internal resistances within the system, including ohmic, activation, bacterial metabolic, and concentration losses. In order to improve the stacking efficiency, unique stake PMFC design was 3D printed and studied, where conductive PLA was used to print the electrodes and normal PLA was used to print the separator. A significant increase of average power from 1.06 × 10⁻⁹ W to 5.47 × 10⁻⁷ W was documented when the individual PMFC was arranged in three-stacked PMFCs connected in series, hinting that the performance of PMFCs was more efficient when they were stacked together. Additionally, PMFCs with a nine-stack parallel connection produced the highest power at 1.62 × 10⁻⁴ W with a power density of 9.18 mW m⁻² and a voltage of 4.54 V. Apart from the superior electrochemical performance, the design of PMFC as watering stakes enhanced the transport of water and oxygen to the plant roots, which were crucial for strong root development. A healthy root environment provides a good supply of rhizodeposits that serve as valuable nutrients for microbes, which allows the biofilm to consume and metabolize in order to produce electrons [24].

Lately, soil microbial fuel cells (SMFCs) have garnered significant interest as an eco-friendly innovation with extensive possibilities in generating renewable energy, treating environmental pollution, and enabling sensing capabilities. In the last 10 years, scientists primarily concentrated on creating SMFCs for low-power sensor surveillance, cleaning contaminated soil, and exploring novel sensing applications [25, 26]. Nonetheless, the main setback of SMFCs originates from the low power output and high operating costs that limit their real field application. In another study, a SMFC reactor was 3D printed using PLA filaments to generate electricity from rice washing wastewater (RWW). This is because RWW represents a huge portion of domestic wastewater, especially in Asian countries and it also contains starch, minerals, and vitamins that are useful for microbial oxidation. 3D printed SMFC with carbon felt electrodes were plugged into the soil with constant addition of 15 mL RWW every 48 hours. Two types of soil (muddy and sandy) were used in this experiment. It was reported that SMFC in muddy soil with RWW had generated a maximum power density of 485.2 mW m⁻², while SMFC in sandy soil only generated 112 mW m⁻² [27]. These results indicated that 3D printed MFCs are able to operate in different types of soil.

3D printing stands out as one of the most optimal choices in creating micro-devices with exact and accurate dimensions [28]. A miniaturized 3D printed MFC reactor enclosure was created to allow oxygen flow at the air-cathode and fuel supply at the anode side. As a result, this miniaturized MFC achieved a maximum power density of 192 μW cm⁻² and a maximum current density of 1.3 mA cm⁻². Furthermore, consistent power generation was observed where repeatable voltage production of 410 mV was recorded, proving that the 3D printed enclosure possessed excellent stability [29]. This means that the enclosure could be reused by just replacing the electrodes on both terminals, thereby providing a cost-effective solution. Similar to the aforementioned work, a small-scale MFC tank with a mixer design on the anode chamber was fabricated via 3D printing as well to promote uniform mixing of the microbial culture and the nutrient substrate. As a result, this high-efficiency mixing area had contributed toward a maximum power density of 199.24 mW m⁻² using graphene-coated carbon cloth as the anode. Moreover, the internal resistance of the system was lowered to only 0.66 kΩ, which indicated a low anode impedance due to the mixing anode chamber design [30].

4. 3D printed MFC separators

Every MFC consists of a separator or membrane that provides a physical separation between the electrodes and in some cases, their electrolytes. This is due to the different reactions of interest at each terminal, which require different electrolytes to suit their conditions. The performance of the separator is influenced by the type of material, thickness, pore size, and surface conditions. Furthermore, membranes help to reduce oxygen and substrate diffusion during microbial oxidation [31], which ultimately helps in improving the MFC’s efficiency. Despite the merits of using a separator, the most commonly used PEM in MFCs such as Nafion 117 membrane suffers from poor conductivity at low humidity and structural changes at high humidity [32], which disrupt the ion transfer process. Hence, the optimization of separators’ parameters can be addressed by using additive manufacturing.

In a novel proton exchange membrane (PEM) development work, TangoPlus acrylate photopolymer resin and natural rubber latex membranes were successfully printed and compared with the commercially available CMI-7000 cation exchange membrane (CEM). Thinner membranes were fabricated by using a 3D printer; the TangoPlus polymer (Figure 4) recorded a membrane thickness of 116 μm and the latex recorded a membrane thickness of 100 μm, which were significantly lower than that of conventional CEM with a membrane thickness of 450 μm. Generally, a thinner membrane is desired as it reduces internal resistance, which enhances proton conductivity [33]. However, this study reported that the peak power production for TangoPlus, latex, and CEM was 0.92, 10.51, and 11.39 μW, respectively. Therefore, it is highly necessary to explore materials with ionically conductive properties so that the ion exchange process can be carried out more efficiently [34].

Figure 4.

3D printed TangoPlus MFC separator. Reprinted with permission from Ref. [33] under an open-access creative commons CC BY license. Copyright 2021, MDPI.

In search of an alternative to the costly Nafion 117 membrane and CMI-7000 CEM, multiple 3D printed separators were synthesized using low-cost extrudable clay as printing materials. Fimo™ air-dry, terracotta air-dry, and terracotta modeling clays were printed using a robotic liquid dispensing machine. The MFCs were then constructed using the 3D printed separators in a membrane electrode assembly (MEA) configuration. The electrochemical studies demonstrated that the Fimo™ air-dry membrane recorded the lowest average surface resistance of 135 Ω, which was even lower than the commercial CEM with a resistance of 143 Ω. Similarly, the terracotta air-dry and terracotta modeling clay membranes possessed slightly higher resistance values of 158 and 169 Ω, respectively. Despite the similar average surface resistance values of all four membranes (150 ± 15 Ω), all three 3D printed ceramic-based membranes produced significantly better results compared to the CEM. Membrane Terracotta air-dry membrane obtained the highest power output at 130 μW followed by Fimo™ air-dry membrane with 111 μW, indicating the superiority of air-dry clay as MFC membranes. Terracotta modeling clay membrane underperformed at 73 μW, and lastly, the commercial CEM only generated 66 μW [35]. In short, separator materials remain a crucial element within the MFC system and require more exploration to identify novel and efficient membrane materials.

5. Conclusion

In essence, the incorporation of 3D printing technology in MFCs has opened avenues for innovation, allowing for meticulous design and customization of every crucial MFC components. While significant progress has been made, continuous exploration of novel materials and optimization of 3D printing parameters in this field ensures further advancements in MFC technology.

Acknowledgments

The authors would like to express their gratitude to the Universiti Kebangsaan Malaysia and Indah Water Research Center (IWRC) for the financial support of this work via Geran Translasi (UKM-TR-010), Dana Pecutan (PP-SELFUEL-2022), and Indah Water Konsortium Sdn. Bhd. (KK-2022-015). Ryan Yow Zhong Yeo would also like to thank Universiti Kebangsaan Malaysia for partially supporting his study through Peruntukan Khas Institut SELFUEL 2022 (BG-H-RA0100-00-0101071).

Conflict of interest

The authors declare no conflict of interest.

Nomenclature

3DP	3D printing
ABS	acrylonitrile butadiene styrene
AM	additive manufacturing
BES	bioelectrochemical system
CAD	computer-aided design
CEM	cation exchange membrane
DIW	direct ink writing
DLP	digital light processing
FDM	fused deposit modeling
GO	graphene oxide
LED	light-emitting diode
MEA	membrane electrode assembly
MET	microbial electrochemical technologies
MSLA	masked stereolithography
MFC	microbial fuel cell
ORR	oxygen reduction reaction
PEM	proton exchange membrane
PLA	polylactic acid
PMFC	plant microbial fuel cell
PPy	polypyrrole
PTFE	polytetrafluoroethylene
RWW	rice washing wastewater
SDG	sustainable development goal
SMFC	soil microbial fuel cell